Sen. Cory Booker of New Jersey announced on Monday that he would break longstanding Senate norms and testify against fellow sitting Sen. Jeff Sessions, President-elect Donald Trump’s US attorney general nominee.

Booker told MSNBC that he would testify against Sessions on Tuesday or Wednesday during the Senate Judiciary Committee hearings surrounding Sessions’ nomination, joining prominent civil-rights activist Rep. John Lewis and the chair of the Congressional Black Caucus, Rep. Cedric Richmond.

“I’m breaking a pretty long Senate tradition by actually being a sitting senator tomorrow testifying tomorrow against another sitting senator,” Booker said. “Please understand I think these are extraordinary times and they call for extraordinary measures.”

Booker cited Sessions’ stances on policing, voting rights, marijuana legalization, and LGBT issues as areas where he disagreed with the Alabama senator, saying Sessions “has a posture and positioning that I think represent a real danger to our country.

“We are at a strike point around issues of policing, around issues of civil rights, around issues of gay and lesbian equality within our country,” Booker added. “We’ve seen consistently Jeff Sessions as Sen. Jeff Sessions voting against everything from the Matthew Shepard Act, voting against – speaking out against key ideals around the Voting Rights Act, taking measures to try to block criminal-justice reform.”

“There’s a whole spectrum of things that Jeff Sessions’ own words represent a real threat to vulnerable populations in this country and is something that I feel necessary to do everything that I can to speak out against,” he said.

Though senators have voted against confirming fellow members to Cabinet positions and expressed opposition during the question-and-answer portion of hearings, Booker’s decision to actively testify against his colleague represents a break with longstanding precedent.

Republicans were quick to decry Booker’s move.

Rep. Chris Collins, one of Trump’s more active supporters on cable TV, described Sessions as a well-respected “Southern gentleman” and dismissed Booker’s testimony as a publicity stunt.

“Cory Booker is all about the latest stunt,” Collins told CNN on Tuesday. “What he’s doing today, never done before in the Senate – it’s not surprising at all that Cory Booker is the one pulling this off.

“What Cory Booker is doing is nothing but being self-serving, grabbing the headlines,” he added.

Many Democrats, including Booker, said they opposed Sessions’ attempts to kill immigration-reform efforts, his stance against the legalization of same-sex marriage, and his support for tough sentences for those convicted of drug crimes.

While Trump’s transition team has said Sessions has a “strong civil-rights record,” many Democrats and civil liberties groups blasted Sessions’ criticism of the Voting Rights Act, which prohibited inhibitions on the right to vote, as well as his criticism of the Justice Department’s civil-rights division, which he said occasionally supersedes its authority and perpetrates “civil wrongs.”

Sessions is also staunchly opposed to marijuana legalization and decriminalization. In April he said, “Good people don’t smoke marijuana.” He has also faced backlash for saying in the 1980s that he thought members of the KKK were “OK, until I found out they smoked pot,” which he said was a joke.